Chapter Focus: Understanding reactions involving aromatic compounds, specifically seven main reactions.
Objectives:
Explore how mono-substituted aromatic reactions proceed.
Extend understanding to multi-substituted benzene ring reactions.
EAS reaction replaces one of the aromatic protons with an electrophile without disrupting aromaticity.
Goal: Add bromine (Br2) to the benzene ring.
Reagents: Catalyst complex serves as an electrophilic agent.
Mechanism Steps:
Nucleophilic Attack: The benzene double bond attacks the bromine electrophile, forming a positively charged intermediate known as the sigma complex.
Energy Requirement: The formation of the sigma complex leads to a loss of aromatic stability, thus requiring a considerable amount of energy.
Proton Transfer: The deprotonation of the sigma complex restores aromatic stability.
Comparison: Unlike addition reactions that destroy aromaticity, EAS preserves it.
Similar to bromination but uses chlorine (Cl2).
Reagents: Specific catalysts are required for creating the electrophilic reagent.
Mechanism:
Nucleophilic attack leads to sigma complex formation.
Proton transfer restores aromatic product.
Mechanism follows the same steps as bromination and chlorination.
Reagents: Iodine in conjunction with suitable catalysts.
Goal: Add sulfur trioxide (SO3) to the benzene ring using fuming sulfuric acid (H2SO4).
Mechanism:
Nucleophilic attack by the benzene ring onto sulfuric acid forms a sigma complex.
Proton transfers restore aromaticity, completing the reaction.
Note: For reverse reactions, dilute sulfuric acid is used.
Goal: Introduce a nitro group (NO2) to the aromatic ring.
Mechanism:
Form a nitronium ion (NO2+) from nitric acid (HNO3) and sulfuric acid (H2SO4).
Proceed with nucleophilic attack and sigma complex formation, followed by proton transfer to restore aromaticity.
Reduction of the nitro group to an amino group via specific reagents can be done afterward.
Goal: Add an alkyl group to the benzene ring.
Reagents: Aluminum trichloride (AlCl3) necessary to generate the carbocation electrophile.
Mechanism:
Alkyl halide reacts with AlCl3 to form a carbocation.
Nucleophilic attack on the carbocation results in a sigma complex.
Proton transfer restores aromaticity.
Considerations:
Possible carbocation rearrangement may occur.
Requires secondary or tertiary alkyl halides; primary can be viable.
Polyalkylation may occur where added groups activate the benzene ring.
Goal: Add an acyl group to the benzene ring.
Mechanism:
Acyl chloride reacts with a Lewis acid to form an acylium ion.
Similar nucleophilic attack and proton transfer steps as other EAS reactions.
Outcome: Forms aryl ketones, which can be further reduced (Clemenson reduction).
Note: Unlike alkylation, polyacylation doesn’t happen as the acyl group deactivates the ring.
Understanding Key Concepts:
Distinguish correct statements about EAS processes.
Identify electrophiles in nitration and other reactions.
Recognize reagents used in halogenation reactions.
Examples:
Identify products formed when benzene undergoes various EAS reactions (bromination, chlorination, nitration, etc.).
Illustrate the use of Friedel-Crafts reactions in synthesis.
Mastery of the outlined reactions is essential for success in organic synthesis.
Continuous practice and revision of mechanisms will enhance understanding and retention.